There are known several string instruments provided with a detecting system for detecting the note played. U.S. Pat. No. 4,635,518 discloses an electronic stringed musical instrument having an electrically insulating fingerboard is disclosed. The fingerboard is provided with a number of segmented frets attached across its upper surface at desired points along its length. Each of the frets includes a number of electrically conducting fret segments each of which are electrically insulated from one another. Any number of strings may be provided on the instrument each string is disposed adjacent to and associated with a single fret segment of each of the segmented frets. A top octave generator and octave dividers are utilized to selectively provide a fret segment of one of the frets with an electrical signal of at least one known referencing frequency. The strings are attached to the instrument in a spaced relationship with respect to the fret segments. Displacing a string to contact one of the fret segments completes an electrical circuit having at least one frequency equal to a frequency of the signal provided to that fret segment. Displacing the same string to contact a different fret segment completes a different electrical circuit having at least one different frequency. Simultaneously depressing a plurality of the strings simultaneously completes a number of electrical circuits each capable of producing a number of different frequencies. The amplitude output of the instrument is dependent upon the voltage applied to each of the strings and is controlled by hand operated transducers.
US2012017748 discloses a digital musical instrument including a fretboard and one or more strings extended over the fretboard. The instrument further includes an electric circuit for generating digital signals based on positions associated with contacts of the strings on the fretboard and a transceiver for transmitting the digital signals to a processing device that generates musical notation based on the digital signals.
U.S. Pat. No. 8,454,418 discloses a game controller having one or more strings is described for a computer gaming application. A plurality of frets can be disposed on a fingerboard and underlying the strings. The frets may include electrically conductive zones that can be electrically insulated from each other, and each zone corresponds to a different string. A polyphonic pickup having a plurality of wire-wound coils coupled to corresponding magnetic returns can be included, and can be adapted to detect striking of at least one of the strings by a user of the game controller. Output signals may be sent from the controller to the gaming application indicative of fingering of the game controller and the time at which the strings of the game controller are struck. Multi-mode apparatus are also described. A stringed apparatus may be used as both a game controller and an instrument.
WO2013109657 discloses an electronic stringed instrument practice device can be configured to perform one or more of the following:    detect when finger positions and/or string to fret contact on a finger or fret board forms an appropriate musical note or musical chord, visually indicate appropriate positions on a finger or fret board for forming a musical note or musical chord, and detect when strings have been selected (e.g., strummed). The electronic stringed instrument practice device can emit sound in the form of musical notes and chords. The electronic stringed instrument practice device can include communication modules for communicating with other computing devices, including mobile phones and tablets. The electronic stringed instrument practice device can interact with applications on other computing devices to further assist users in learning how to play a stringed musical instrument.
US2013247744 discloses a stringed instrument is equipped with an electrical conductor electrically connected to the frets mounted in the fretboard of said stringed instrument. Said stringed instrument is also equipped with a power source, light emitting members in electrical contact with the strings of the instrument (in one embodiment light emitting diodes) and electrical conductors electrically connecting together the components of the invention. By means of pressing down anyone of the strings capable of transmitting electric current against anyone of the frets capable of transmitting electric current connected to the electrical circuit comprised of said electrical components, said circuit closes and the light emitting member(s) associated with the string that is pressed down against the fret is lit.
JP2009271484 discloses a sensing means for sensing contact/non-contact is constituted by making a string and a fret in an electricity conductive state, and predetermined light is generated by a performance means interlocking with the sensing means, in a performance device for the string instrument.